1. Field of the Invention
The present invention relates to wireless communication and, more particularly, to a method and apparatus for reporting a channel state in a wireless communication system supporting multiple carriers.
2. Related Art
In order to respond various forums and new technologies relating to the fourth generation mobile communication, the 3GPP, which establishes the technical specifications of the third generation mobile communication system, begins research on long term evolution/evolved packet core (LTE/EPC) technologies from the end of 2004, as part of efforts to optimize and improve the performance of the 3GPP technologies. The EPC, which is advanced based on 3GPP SA WG2, is a study on network technology which aims to determine a structure of a network in parallel with the LTE task of 3GPP TSG RAN and support the mobility between heterogeneous networks, and is one of the important standardization issues of the latest 3GPP. This is a task aiming to develop the 3GPP system into a system supporting a variety of IP based wireless access technologies, which has been in progress in order to achieve the optimized packet-based system that minimizes transfer delay with further improved data transfer capacity.
Hereinafter, technical terms used in the description of the present invention will be described.                An MTC function is a function for supporting communication between MTC devices or communication between an MTC device and an MTC server, and means that communication is made without human intervention in the middle unlike existing person-to-person connection. For example, MTC application includes communications between automatic vending machines and servers, point of service (POS) devices and servers, and electricity and water probes and servers. Herein, a corresponding device is called an MTC device. Here, MTC is also called as the name of Machine to Machine or machine communication.        A tracking area (TA) indicates an area where E-UTRAN provides service and includes one or a plurality of E-UTRAN cells.        A routing area (RA) indicates an area where GERAN/UTRAN provides service and includes one or a plurality of GERAN/UTRAN cells.        A list of tracking area identities (TAIs) that identify the tracking areas that the UE can enter without performing a tracking area updating procedure. The TAIs in a TAI list assigned by an MME to a UE pertain to the same MME area. A detailed description on the TAI list is cited from part disclosed in standard document 3GPP TS 24.301 v9.1.0.        Mobility management entity (MME) area: the MME area is the part of the network served by an MME. An MME area consists of one or several tracking areas. All cells served by an eNodeB are included in an MME Area. A detailed description on the MME area is cited from part disclosed in standard document 3GPP TS 23.002 v9.2.0.        UMTS: is a universal mobile telecommunication system and indicates 3G network.        EPS: is an evolved packet system and is the general term for evolved packet core, various access network, and terminals, which support the LTE access network. Additionally, the EPS is the network of an evolved form of the UMTS.        NodeB: is installed in outdoor as a base station in a UMTS network and its coverage is a macro cell scale.        eNodeB: is installed in outdoor as a base station in an EPS network, and its service coverage is a micro cell scale.        UE: is User Equipment and indicates a terminal device.        IMSI: is international mobile subscriber identity and is the id of a user internationally uniquely assigned in a mobile communication network.        SIM card: is a subscriber identity module and includes user subscriber information such as IMSI.        UICC: is universal integrated circuit card and is used as the same meaning as a SIM card.        MTC: is machine type communication and indicates communication between devices without human intervention.        MTC device: is an UE performing a specific purpose with a communication function via a core network, and includes vending machines and probes, for example.        MTC server: is a server on network, which manages an MTC device and exchanges data. It may exist outside a core network.        MTC application: is actual application using an MTC device and an MTC server and includes remote metering and shipments moving track, for example.        MTC feature: according to functions or features of a network for supporting MTC application, that is, the purpose of each application, some features are required. For example, there are MTC monitoring (required for remote metering when equipment is lost) and low mobility (almost no mobility in the case of a vending machine).        RAN: is a radio access network and is the general term for 3GPP wireless access such as RNC, NodeB, and eNodeB.        Home location register (HLR)/home subscriber server (HSS): is a DB representing subscriber information in a 3GPP network.        RANAP: is an acronym for radio access network application part and indicates an interface between nodes (MME/SGSN/MSC) for controlling a RAN and a core network.        Cell camping (or camp) on state indicates a state when a terminal completes a cell selection/reselection process and selects a cell. A detailed description on this is cited from part disclosed in standard document 3GPP TS 36.304 v9.1.0.        Idle mode signaling reduction (ISR) is a service to improve the efficiency of network resources by reducing signaling for position registration when a terminal moves in different access networks such as an E-UTRAN network and a UTRAN/GERAN.        IMS centralized services (ICS) provides a stable consistent service to an IMS, regardless of an access network having a terminal attached (that is, even if it is attached to an IP-CAN and also a CS domain). A detailed description on the ICS is cited from part disclosed in standard document 3GPP TS 23.292 v9.4.0.        IP multimedia subsystem (IMS) is a system providing an IP based multimedia service.        Attach refers to that a terminal accesses a network node, and also refers to attach occurring during handover in a broader sense.        
From now on, the present invention will be described referring to the above described technical terms.
FIG. 1 is a conceptual diagram illustrating a 3GPP service model for MTC support.
In order for MTC support, it is defined that communication is made via a PS network in the GSM/UMTS/EPS of the 3GPP standard but the present invention describes how to apply for a CS network. Definition on a network structure in the current technical specifications suggests the use of the existing bearer of the 3GPP. A method of using short message service (SMS) in order to exchange data between an MTC device and an MTC server is suggested as one of an alternative solution. Because of the nature of MTC application, the use of SMS is suggested in consideration that a small amount of digital data such as metering information or product information become its target, and in that way, existing SMS and IMS based SMS supports are available. Referring to FIG. 1, MTCsms is a data exchange interface through existing SMS, and MTCi is a data exchange interface for 3GPP bearer service and IMS.
Hereinafter, typical congestion state control (such as overload control or congestion control) in MTC service will be described.
An example of the case that a congestion state occurs in MTC is as follows: when incorrect functions of an MTC server or MTC application program are performed; when an external event causing a large number of MTC devices to access occurs; when a specific program is configured to operate repeatedly at a specific predetermined time. In such cases, it is very difficult to implement an MTC application program to operate in reality in linkage with a core network. This is because nodes (SGSN/MME/PGW, etc) in a core network are easily damaged by traffic concentrating data.
Accordingly, a method of controlling a congestion state caused by typical core network nodes is as follows.
A) A network node may deny a specific access. The limit standard for denying the access is available according to an APN or MTC group. Or, an MTC device may access a network node only at a predetermined time. However, if there is a congestion state, a network node transmits back off time to an MTC device, so that the network node provides to the MTC device information that prevents an access of the MTC device during that period. At this point, the network node may set back off time through a randomization method.
However, when a control is made on a specific APN or group, a state may be normal in a core network but a congestion state may occur in a specific base station area. Additionally, the performance of the randomization is determined according to a range of an applied time relating to an access between an MTC device and a core network. At this point, if the range of the applied time is short, randomization effect becomes less and this is due to peak. On the other hand, if the range of the applied time is too long, MTC service may be received past the intended time. Additionally, if a new target comes in, an entire calculation is needed again. In this case, since delay time needs to be sent to each MTC device (that is, a terminal) again, large signaling overhead is an issue. Furthermore, if delay time is not sent to an MTC device, this affects performance in terms of technical limitations.
B) Before an MTC device is used at a predetermined time, a position updating process of TAU/RAU is performed on a cell that the MTC device belongs. However, this is to register an accurate position registration before use due to long-term no use. In this case, since it is a long-term off state, position information needs to be updated during an access. This may not be the solution of the fundamental problem.
C) An access start time that an MTC device accesses a network node within a predetermined time is randomized. This is to prevent traffic from concentrating at a specific time. This method may trigger an MTC server or an MTC device. However, when an MTC server or an MTC device is triggered, congestion may occur because it does not take a 3GPP network situation into account.
In this case, it is very difficult to take a core network situation into account and even if it has the effect for a short time, since it does not consider future events to occur, errors are accumulated, so that it is likely to affect the future situation. Accordingly, according to a configuration of an MTC server or an MTC device, a congestion situation may be caused. Additionally, when a new target comes in, this may cause the technical limitation such as the above A).